user6174 Is the private space encrypted when closed ?
No, it isn't. Once unlocked after a reboot, all files will stay available in unencrypted form. You need to actually restart your phone for it to go back to being only in encrypted form. The GrapheneOS project did an attempt to fix this a few months back, so that locking the private space would make the files and app data go back to encrypted form, but it didn't work out any well so was reverted. I guess a new fix might be coming soon.
user6174 What use is it if the phone is not shared with others (I'm the only user) ?
You can keep some files and app data password protected and encrypted even if the rest of your phone in unlocked, in case your phone is taken from you. The private space can also be used to isolate apps from each other, so the apps outside the private space cannot talk to the apps within the private space, and the other way around.
user6174 Can it be compared, in a way, to a Veracrypt container? It seems to work in a similar way, am I wrong?
It should be far more secure than a VeraCrypt container. A VeraCrypt container does not prevent metadata from leaking and being stored outside the container, and does not isolate apps from within and outside the container from talking to each other. The private space does both of those things.